The present invention relates to a method of detecting trace amount of electrically conductive substance which may exist in glass fibers or products made from glass fibers, as well as to an apparatus for carrying out such a method.
Glass fibers often contain foreign matter which is included in the raw materials or brought into the glass fibers during the production. Although there are many kinds of foreign matter, containment of conductive matter should be preferably avoided because such matter, even if the amounts thereof are very small, seriously impair the electric insulating power of the glass fiber products when the latter are used as insulating materials. Examples of these conductive foreign matter often found in glass fibers are metals such as gold, silver, copper and iron; metallic compounds such as nickel sulfide and iron sulfide; and alloys such as platinum-rhodium, duralumin and gun metal. Glass fiber insulating material find uses as, for example, insulation and reinforcement of printed circuit boards. In recent years, there is a trend towards higher density and quality of printed circuit boards and, accordingly, the reduction of insulating power due to containment of conductive matters in glass fibers is becoming matter of greater significance. Under this circumstance, there is an increasing demand for development of method and apparatus for detecting trace amounts of conductive matter contained in glass fibers.
Japanese Patent Laid-Open No. 61217/74 discloses a method of detecting conductive foreign matter in glass sheet. In this method, the conductive foreign matter in the glass sheet is heated by induction heating and the presence of such matter is detected through measurement of temperature rise. Unfortunately, however, this method cannot be applied satisfactorily to the detection of trace amounts of conductive foreign matter in glass fibers, because it offers only low accuracy detection.
A detecting method employing a so-called metal detector is well known. This device has a coil which forms a high-frequency magnetic field. In operation, the object to be examined is placed in this magnetic field. If the object contains some foreign matter, the magnetic field is disturbed to induce a voltage in the receiving coil, so that the presence of the conductive matter can be known by a measurement of the voltage in the receiving coil. This method, however, is effective only to find spherical metallic bodies of diameters not smaller than 100 .mu.m, and cannot be used effectively for the detection of minute conductive matters of about 1 .mu.m diameter contained in a single glass fiber of about 10 .mu.m diameter of products woven from glass fibers.